Cosette Weinberg is the first daughter of Ludwig Parvis.
Cosette is a very beautiful lady. She has a fair, unblemished face and clear, beautiful features. In the novel, she has silver hair which resembles moonlight, and in the manhwa, she has blonde hair. Her eye color and hair color were inherited from her father in both novel and manhwa, and she's a petite woman. She has red eyes like her father and wavy hair like her mother.
Cosette was born as Rowena and Ludwig's first child, but none knew of her existence until she was introduced in Zekehart's birthday party. She was born after her mother got kicked out of the Parvis family after being divorced, but her life was in danger because of the assassins hired by the Edinburgh family. Cosette was just a baby back then, and her mother was desperate to protect her.
Thus, her mother sacrificed her soul by making a contract with a demon to protect Cosette from the assassins and to get revenge on Ludwig and the Edinburgh family. Hence, the baby Cosette was possessed by the demon, and she was left in her uncle's care after her mother's death. But the real Cosette died the moment the demon possessed her, and the demon hatched it's plan to bring the whole country into ruins.
Cosette was introduced by Isaac Weinberg at Zekehart's birthday party as the Duke's 'real' daughter, and he chose to do it at his party to get as much attention as possible from the nobles. Ludwig agreed to Cosette's request of proving herself as his daughter since all that mattered to him was who inherited his blood. Thus, Cosette was allowed to live in the Parvis estate despite her past being a clean slate even after much investigation.
Everything about her was suspicious since a commoner like her was able to naturally adapt to noble etiquette, and she also acted like a noble lady without any prior training. When asked why her existence wasn't revealed all these years, Isaac claimed that she was raised by a commoner couple deep inside the woods to protect her which made him difficult to find her all these years.